In many communication applications, one is required to realize a time variant multi-rate interpolation filter followed by a time invariant filter, or a time invariant filter followed by a time variant multi-rate decimation filter. For these two filter configurations, the direct realization of the time invariant filter is computationally intensive because it operates at a higher sampling rate as compared to the sample rate of the input of the interpolation filter or the output of the decimation filter. In order to reduce the computational cost of the fixed filter, the time invariant filter can be combined with the time variant multi-rate interpolation/decimation filter in such a manner as to have the combined filter effectively operate at the lower sampling rate.
Such filtering techniques find practical application in the area of multiple access interference (MAI) mitigation. In direct sequence code-division multiple access (DS-CDMA) systems, MAI is a factor that contributes to a limitation in system capacity and performance. In an attempt to reduce the effect of this factor, one can employ some form of multi-user detection (MUD) algorithm. The basis of a typical MUD detector is the application of information known about other users to improve detection of each individual user.
Of particular interest is the class of MUD detectors known as subtractive interference cancellation detectors. The fundamental principle behind these detectors is that an estimate is made of each individual user's contribution to the total MAI and then subtracted out from the received signal such that the MAI affecting each individual user is reduced. Typically, the detection process is carried out in an iterative manner, where the data decisions of the previous iteration are used as the basis for the next iteration's MAI estimates.
For a chip-rate interference cancellation detector, the process of calculating each individual's contribution to the total MAI involves “regenerating” chip-rate samples from symbol rate data estimates. As an example, for the time division duplex (TDD) mode of a Universal Mobile Telecommunications System terrestrial radio access (UTRA) communication system, regeneration for a given user involves: spreading by the user's corresponding real channelization code, scrambling with a cell-specific complex code, and filtering with the user's channel estimate, which typically requires 28 to 64 complex taps. Note that the spreading codes generally vary in length from 1 to 16. However, the cell-specific scrambling code usually has a fixed length of 16. Therefore, if spreading and scrambling are combined into a single function, the result is effectively a time variant multi-rate interpolation filter. In this way, the regeneration process can be considered as a time variant multi-rate interpolation filter cascaded with a time invariant filter.
Given that regeneration is performed on a per cancellation iteration basis for each user, the computational cost of regeneration is substantial. As such, there is a need to reduce the computational cost of regeneration without degrading the accuracy of the regenerated results. Conventionally, regeneration is accomplished by first performing channelization in the traditional manner, such as by effecting a single, time variant complex multiplication at the chip rate. Finite impulse response (FIR) filters, whose complex coefficients are given by each user's corresponding channel estimate, follow. Since these FIR filters are operating at the chip-rate, the computational requirements are very high.
As an example, consider a TDD mode UTRA system with eight active users and channel estimates with 57 complex taps. The total computational cost of regeneration would be 464 complex calculations per chip-rate period, per cancellation iteration. Assuming that interference cancellation uses four cancellation iterations per TDD slot period (equivalent to 2560 chip-rate periods or 667 microseconds), this corresponds to a computational requirement of 7.2 billion complex operations per second. An alternative technique that has been proposed involves the cancellation of interference at the symbol-rate as opposed to the chip-rate. This approach eliminates the need for chip-rate regeneration, but also may result in a reduction in the accuracy of the MAI estimates.
Accordingly, there is a need for efficient realization of a time variant multi-rate interpolation, or decimation FIR filter, cascaded with a time invariant FIR filter.